1. Field of the Invention
This invention relates to the field of ultrasonic imaging, and, more specifically, to the use of liposomes in ultrasonic imaging procedures.
2. Background of the Invention
There are a variety of imaging techniques which have been used to detect and diagnose disease in animals and humans. One of the first techniques used for diagnostic imaging was X-rays. The images obtained through this technique reflect the electron density of the object being imaged. Contrast agents such as barium or iodine are used to attenuate or block X-rays such that the contrast between various structures is increased. For example, barium is used for gastrointestinal studies to define the bowel lumen and visualize the mucosal surfaces of the bowel. Iodinated contrast media is used intravascularly to visualize the arteries and this is called angiography. X-rays, however, are known to be dangerous. The radiation employed in X-rays is ionizing and the deleterious effects of the ionizing radiation are cumulative.
Magnetic resonance imaging (MRI) is another important imaging technique, however it has the drawbacks of expense and the fact that it cannot be conducted as a portable examination. In addition, MRI is not available at many medical centers.
Radionuclides, employed in nuclear medicine, provide another imaging technique. In employing this technique, radionuclides such as technetium labelled compounds are injected into the patient, and images are obtained from gamma cameras. Nuclear medicine techniques, however, suffer from poor spatial resolution and expose the animal or patient to the deleterious effects of radiation. Furthermore, there is a problem with the handling and disposal of radionuclides.
Ultrasound, another diagnostic imaging technique, is unlike nuclear medicine and X-rays in that it does not expose the patient to the harmful effects of ionizing radiation. Moreover, unlike magnetic resonance imaging, ultrasound is relatively inexpensive and can be conducted as a portable examination. In using the ultrasound technique, sound is transmitted into a patient or animal via a transducer. When the sound waves propagate through the body, they encounter interfaces from tissues and fluids. Depending on the reflectivity and acoustic properties of the tissues and fluids in the body, the ultrasound sound waves are either reflected or absorbed. When sound waves are reflected by an interface they are detected by the receiver in the transducer and processed to form an image. The acoustic properties of the tissues and fluids within the body determine the contrast which appears in the resultant image.
Advances have been made in recent years in ultrasound technology. However, despite these various technological improvements, ultrasound is still an imperfect tool in a number of respects particularly with respect to the detection of disease in the liver and spleen, kidneys and vasculature and in measuring blood flow. The ability to detect and measure these things depends on the difference in acoustic properties between blood or other tissues and the surrounding tissues. As a result, contrast agents have been sought which will increase the acoustic difference between blood and surrounding tissues in order to improve the measurement of blood flow, or between one tissue and another such as between the liver and a tumor in order to improve disease detection.
The principles underlying image formation in ultrasound have directed researchers to this pursuit of contrast agents. When sound waves from ultrasound pass through a substance, the acoustic properties of that substance will depend upon the velocity of the sound and the density of that substance. Changes in the acoustic properties or acoustic impedance of the substance are most pronounced at interfaces of different substances with greatly different density or acoustic impedance, particularly at the interface between solids, liquids and gases. When the ultrasound sound waves encounter such interfaces, the changes in acoustic impedance result in a more intense reflection of sound waves and a more intense signal in the ultrasound image.
Many of the prior art contrast agents developed to date for ultrasound have comprised liquids containing microbubbles of gas where the microbubbles have been encapsulated with gelatin or saccharine. Those microbubble and gelatin/saccharine constructs have most often been prepared using agitation techniques. Other prior art is directed to attempts with protein-associated air bubbles or air bubbles incorporated in microspheres composed of either albumin or collagen. Furthermore, heavy metal particulates have been evaluated as ultrasound contrast agents. There have also been some reports of liposomes described as useful in ultrasonic applications having gas or gaseous precursors encapsulated therein.
While the prior art has produced some ultrasound contrast agents which are echogenic on ultrasound, that is, provide a contrast enhancement signal, the contrast agents developed thus far have various problems. The protein based air bubble systems have the drawback that a foreign protein which may be antigenic and potentially toxic is being employed. The liposomal contrast agents have had problems with uneven size distribution and poor stability. The gaseous precursor containing liposomes have also been inefficient in their ability to form contrast enhancing gas in vivo. Moreover, while some of the prototype prior art contrast agents have demonstrated echogenic effects as transpulmonary vascular contrast agents, many of these agents have failed to demonstrate a convincing effect on improving tumor imaging in, for example, the liver or spleen. Furthermore, many of the methods for preparing these ultrasound contrast agents, particularly the gas encapsulated liposomes, are inefficient, expensive, and otherwise unsatisfactory.
The present invention is directed to answering these and other important needs.